1. Field of the Invention
The present invention relates to a scanning electron microscope, and particularly to a scanning electron microscope which controls an electric current of electrons irradiated on a sample.
2. Description of the Prior Art
A scanning electron microscope is an apparatus for forming a scanned image of a sample by: narrowly focusing a beam of electrons generated by an electron source by use of condenser lenses and an object lens; thus irradiating the focused electron beam on the sample while two-dimensionally scanning the focused electron beam over the sample; thereby detecting secondary electrons generated from the sample; and processing a signal resulting from the detection in synchronism with the scanning of the electron beam.
Examples of this type of scanning electron microscope includes a scanning electron microscope which is provided with an aperture for preventing an electron beam from scattering between a first condenser lens and a second condenser lens, and with an object lens aperture at a stage posterior to the second condenser lens, for the purpose of controlling an amount of the electron beam (see Japanese Patent Application Laid-open Publication No. Hei. 8-241689 (FIG. 6)).